The present invention relates to an objective optical element that converges a light flux on an information recording surface of an optical information recording medium and to an optical pickup device.
With practical use of a short wavelength red laser in recent years, there has been manufactured DVD (digital video disc) representing a high density optical information recording medium (which is also called an optical disc) which is the same as CD (a compact disc) in terms of a size and has a large capacity.
Further, in addition to the aforementioned CD and DVD, there have been commercialized optical discs in various standards each having a different wavelength of a light source and a different thickness of a protective base board such as, for example, CD-R, RW (write-once compact disc), VD (video disc), MD (mini-disc) and MO (magneto-optical disc).
Further, an attempt for the shorter wavelength of a semiconductor laser has been advanced, and research and development have been advanced for a high density optical disc (hereinafter referred to as “high density DVD”) with a protective base board-thickness of 0.1 mm employing a violet laser semiconductor laser light source having a wavelength of about 400 nm and an objective lens in which the image-side numerical aperture (NA) is enhanced to about 0.85 and for high density DVD with a protective base board thickness of about 0.6 mm employing an objective lens whose image-side numerical aperture (NA) is about 0.65.
There have been proposed a various types of optical pickup devices each having the so-called compatibility to converge two types of light fluxes each having a different wavelength with a single objective lens on an information recording surface of each of two types of optical discs.
As an optical pickup device having compatibility, there is known an optical pickup device wherein a ring-shaped zonal or grid-shaped diffractive structure is formed on a surface of an objective lens and on a surface of an optical element arranged separately from the objective lens.
However, in the optical pickup device for compatibility provided with the diffractive structure of this kind, it has been difficult to satisfy sine conditions for correcting spherical aberration and wavefront aberration for both of DVD and CD and for correcting off-axial coma, even when utilizing diffracted ray by diffracting actions, because a thickness of a protective base board is not the same and a wavelength of a light flux to be used is not the same, for two types of optical information recording media (for example, DVD and CD).
As a cause for the foregoing, the diffracting action stated above is usually used to correct spherical aberration that is caused when each of two types of light fluxes used respectively for DVD and CD each being different in terms of wavelength passes through a refracting lens, and it is known that an angle (an angle of incidence and an angle of emergence) of a ray of light caused by the diffracting action is dependent on a pitch of the diffractive structure (see the following expression).n×sin θ−n′×sin θ′=m×λ/P(n represents a refractive index of a medium on the incidence side, θ represents an angle of incidence, n′ represents a refractive index of a medium on the emergence side, m represents an order of diffraction, λ represents a wavelength and P represents a pitch (diffraction pitch))
Further, a difference of an angle of emergence between DVD and CD is expressed by the following expression.−n′dvd×sin θ′dvd−n′cd×sin θ′cd=m×dλ/P(n′dvd and sin θ′dvd−n′cd represent respectively a refractive index and an angle of emergence of a medium on the emergence side in DVD, n′cd and sin θ′cd represent respectively a refractive index and an angle of emergence of a medium on the emergence side in CD, m represents an order of diffraction, dλ represents a difference of a light flux wavelength between DVD and CD and P represents a pitch (diffraction pitch))
Therefore, when a diffractive structure is formed only on an optical surface (plane of incidence) on one side of an objective lens, for example, diffraction pitch P necessary for correction of spherical aberration is determined by a difference of a thickness of a protective base board between DVD and CD and by a difference of a wavelength between two types of light fluxes. Therefore, a diffractive structure having a sufficient difference of an angle of a ray of light for correcting a sine condition is not obtained. Even if the sine condition is corrected sufficiently, there still remains a problem that correction of spherical aberration turns out to be insufficient or excessive.
Further, an optical pickup device wherein a diffractive structure is formed on each of a plane of incidence and a plane of emergence of an objective lens, for example, is also known (for example, see Patent Document 1).
In Patent Document 1, there is disclosed an optical pickup device wherein two surfaces of an objective lens including a plane of incidence and a plane of emergence are constructed to be a diffractive surface, and aberration is corrected for different wavelengths and different base board thicknesses by image forming ability in substantial diffraction limit shown by diffracted ray of the same order for two optical discs each having a different cover glass thickness and for two different wavelengths corresponding to the optical discs.
(Patent Document 1)
TOKKAI No. 2001-147367
(Problems to be Solved by the Invention)
However, the device disclosed in Patent Document 1 is one wherein diffracting actions, namely, functions to correct spherical aberration obtained by a diffractive structure provided on only one side of a conventional objective lens are shared by two diffractive structures by providing them respectively on both sides of an objective lens.
Accordingly, there has been a problem that coma and astigmatism caused in the case of an image height caused by tracking or the like cannot be corrected for two types of optical information recording media.